The solution of problems raised by the wedging of the upper support plate carrying the guide tubes of the fuel clusters in relation to the reactor vessel presents special difficulties, both because of the considerable diametral dimensions of this plate and owing to the need for maintaining, by way of the bearing means used, suitable circulation of the pressurized cooling fluid of the core contained in this vessel.
One solution known in the art involves simply mounting, between the support plate and its bearing; surface on the flange of the core casing or the vessel cover, a ring slightly skewed on itself in the manner of an elastic washer, to form a conical spring abutting the two mutually confronting surfaces. Such a ring achieves the desired purpose of wedging, while at the same time affording satisfactory sealing between the cover of the vessel and the region provided between the latter and the support plate. In this case, however, contact between the bearing surfaces on the ring is made over a very narrow circular band, thus generating a high pressure on a very small bearing surface, thereby risking permanent deformations of the components in contact, with the need to replace these frequently, especially when they are damaged by the edges of the spring. There are also flow-adjusting devices consisting of stationary nozzles which are difficult to produce and which it is impractical to inspect and change, if so required. Moreover, this ring, because of its large diameter and thickness, necessitates means of production and of handling which are costly, bulky and difficult to operate, the difficulties increasing when this component has to be replaced once it is contaminated.
Another solution consists in ensuring the wedging of the support plate by means of an assembly of individual cartridges distributed uniformly about the axis of the vessel, each of these cartridges being formed from a stack of elastic washers, of the preloaded Belleville washer type, which are contained in a receptacle of the plate and which are retained by a cover screwed thereonto, this, a piston, which supports the flange of the core casing and which compresses the washers by means of a bearing collar integral with this piston, moving in this receptacle. Such an embodiment, described particularly in FR-A-2,407,384, also has the disadvantage, that, if the force exerted on the piston exceeds the load opposed by the washers, the bearing of the support plate is falsified, the cartridges no longer ensuring the desired wedging. To alter the seating of the support plate, the screws retaining the cover of the receptacle must then be removed for the purpose of acting on the stack of washers, and this operation, which must be carried out in a controlled atmosphere, increases the risks, especially of breakage of the normally blocked screws. This solution is therefore inconvenient and difficult to put into practice.
Finally, EP-A-0,138,477 discloses a structure for wedging the upper support plate of the cluster guides of a nuclear reactor in relation to the vessel, using an assembly of elastic washers, of the Belleville washer type, which are mounted between two suitable bearing surfaces provided between the support plate and a flange formed on the upper part of the cylindrical casing containing the reactor core. However, in such a solution, the stack of washers is loaded by a hollow bush which passes through this stack, this bush having a shoulder for bearing on the washers and being mounted slidably in a countersink provided in the flange. A sealing sleeve allows the cooling fluid to communicate with the regions located above and below the flange of the core casing. The disadvantage of such a solution is that the bush moves in relation to the flange, thus risking causing wear of the latter and falsifying the suitable wedging of the support plate in relation to the flange. Moreover, the sleeve ensuring the sealing comprises a metal bellows which inadequately withstands the ambient environment and especially the pressure of the fluid passing through the sleeve.